Due to their high storage density, long data retention life, and relatively low cost, optical disks have become the predominant media format for distributing information. Large format disks, and more recently, DVD disks, have been developed for storing full length motion pictures. The compact disk (CD) format was developed and marketed for the distribution of musical recordings and has replaced vinyl records. High-capacity, read-only data storage media, such as CD-ROM and DVD-ROM, have become prevalent in the personal computer field, and the DVD format may soon replace videotape as the distribution medium of choice for video information.
Recently, relatively inexpensive optical disk writers and writable optical media have become available, making optical disks popular as backup and archival storage devices for personal computers. The large storage capacity of writable optical disks also makes them ideal for use in multimedia authoring and in other applications which require access to large amounts of storage. Current writable optical disk technologies include several write-once technologies, such as CD-Recordable (CD-R) and DVD-Recordable (DVD-R); a few technologies permit writing, erasing, and rewriting data on a disk, such as Mini-Disk (MD), which uses magneto-optical technology; still others use phase-change and dye-polymer technology. Recent advances in writable optical disk technology have made rewritable optical media more practical, and the specification for DVD-RAM calls for use of high-capacity rewritable optical media.
An optical disk is made of a transparent disk or substrate in which data, in the form of a serial bit-stream, are encoded as a series of pits in a reflective surface within the disk. The pits are arranged along a spiral or circular track. Data are read from the optical disk by focusing a low power laser beam onto a track on the disk and detecting the light reflected from the surface of the disk. By rotating the optical disk, the light reflected from the surface of the disk is modulated by the pattern of the pits rotating into and out of the field of laser illumination. Optical and imaging systems detect the modulated, reflected, laser light and produce an electrical signal that is decoded to recover the digital data stored on the optical disk.
Data is typically recorded on writable optical disks by using a higher power laser than is used for reading. The media for use with optical disk writers typically includes a recording layer, made of a material which changes its optical characteristics in response to the presence of the beam from the high power laser. The high power laser is used to create "pits" in the recording layer which have a different reflectivity than surrounding areas of the disk, and which can be read using a lower power reading beam. In systems having the ability to erase and re-record data, a laser having a power output between the low power used for reading and the high power used for writing may be used to erase data. Alternatively, some systems employ a laser which outputs a different wavelength of light to erase data from the optical media. The methods used to write and erase optical disks depend on the type of recordable media being used.
To write or retrieve data from an optical disk, the foregoing optical systems include a pickup assembly which may be positioned to read or write data on any disk track. Servo mechanisms are provided for focusing the optical system and for keeping the pickup assembly positioned over the track, despite disk warpage or eccentricity.
Because in most previously known systems the data are read from the disk serially, i.e. one bit at a time, the maximum data transfer rate for an optical disk reader is determined by the rate at which the pits pass by the pickup assembly. The linear density of the bits and the track pitch are fixed by the specification of the particular optical disk format. For example, CD disks employ a track pitch of 1.6 .mu.m, while DVD employs a track pitch only about one-half as wide.
Previously known methods of increasing the data transfer rate of optical disk readers and writers have focused on increasing the rate at which the pits pass by the pickup assembly by increasing the rotational speed of the disk itself. Currently, constant linear velocity (CLV) drives with rotational speeds of up to 16.times. standard speed are commercially available, and even faster reading speeds have been achieved using constant angular velocity designs. Higher disk rotational speeds, however, place increasing demands on the optical and mechanical subsystems within the optical disk player, create greater vibration, and may make such players more difficult and expensive to design and manufacture. Higher rotation speeds also make accurately writing data to a disk more difficult, so few CD-R systems are available that record at faster than 4.times. the standard speed.
A cost effective alternative to increasing the disk rotational speed to provide faster optical disk readers is to read multiple data tracks simultaneously, as described in commonly assigned U.S. Pat. No. 5,426,623 to Alon et al. In accordance with the methods and apparatus provided therein, for example, ten adjacent data tracks may be read simultaneously. Thus, even if the disk is rotated at only 4.times. the standard speed, the capability to read ten tracks simultaneously provides the equivalent of a 40.times. drive.
It should be noted that as used herein, a data track is a portion of the spiral data track of a typical optical compact disk which follows the spiral for one rotation of the disk. Thus, a drive capable of reading multiple data tracks simultaneously reads multiple such portions of the spiral data track at once. For optical disks having concentric circular tracks, a data track would refer to one such circular track. For disks having multiple concentric spiral tracks, such as those described in commonly assigned, copending U.S. patent application Ser. No. 08/885,425, a data track would refer to one of the concentric spiral tracks.
One way in which a drive capable of reading multiple data tracks simultaneously may be implemented is through use of multiple beams, arranged so that each beam illuminates a single data track on the disk. U.S. Pat. No. 5,144,616 to Yasukawa et al. shows a system in which multiple laser diode emitters are used to provide multiple beams. Other methods may also be used to provide multiple beams. U.S. Pat. No. 4,459,690 to Corsover, for example, describes a multi-beam system in which an illumination beam generated by a single laser source is split into multiple beams using an acousto-optic device that dithers the beam in a direction normal to the track direction.
The beams in a multi-beam optical pickup may also be provided by using a diffractive element to split a single beam into multiple beams. This technique is used to generate the beams in a three-beam tracking system, as shown in The Compact Disc Handbook, Pohlmann, K., 2nd ed., A-R Editions, 1992, pp. 108-115. In commonly assigned, copending U.S. patent application Ser. No. 08/911,815, a diffractive element is used to split an illumination.beam into a plurality of reading beams. Through careful design, it is possible to produce a diffractive element capable of generating multiple beams having the proper spacing to align with the data tracks of an optical disk.
These methods of increasing the speed by handling multiple tracks at once have generally only been used for optical disk readers, since writing multiple tracks of an optical disk simultaneously presents greater challenges. For example, whereas an optical disk reader which reads multiple tracks simultaneously must provide illumination for multiple tracks, an optical disk writer which writes multiple tracks simultaneously must control multiple lasers, which must be individually modulated according to the data being written.
Optical disk writers and readers are typically combined, so that the same drive may be used both to read and write optical disks. Such drives, however, are generally not as fast at reading optical disks as are typical optical disk readers. Many of the enhancements which increase the speed of optical disk readers are difficult to apply in a system which also writes optical disks. Fast CD-R drives are typically capable of recording at 4.times. speed and reading at 8.times. speed, while CD-ROM readers with speeds of 16.times. and faster (using constant linear velocity) are readily available. Consumers are thus left with a choice of purchasing a high speed CD-ROM reader, or a relatively low.speed drive which can both read and write CD-ROMs.
It would therefore be desirable to provide an optical drive with the capability of reading optical disks at high speed, and of writing optical disks with at least the same speed as current optical disk writers.
It also would be desirable to provide an optical drive with the capability of reading optical multiple tracks of an optical disk simultaneously, while also providing the capability to write data to an optical disk.